1. Field of the Invention
This invention relates to an oxidation resistant steel and method for making same, and more specifically, this invention relates to an oxidation resistant steel and a method for making oxidation resistant steel having superior resistance at 700xc2x0 C. and at temperatures exceeding 700xc2x0 C.
2. Background of the Invention
Oxidation rates and mechanisms in numerous metals and alloys have been investigated in various environments. Oxidation characteristics of iron-based materials depend on the alloying elements of those materials and the oxidation environments.
Additions of chromium, manganese, silicon, and aluminum singly or in combination can promote the formation of protective films, such as MO2 oxide, M3O4 spinels and M2O3 sesquioxides (where M=Fe, Cr, Mn, Si and/or Al).
Iron base alloys with additions of 13 percent or more chromium are designated stainless steels and in oxidizing environments form a strongly adherent protective chromium oxide over the base metal.
Aluminum and silicon are frequently added to stainless steel in an attempt to improve protective oxide films. In Fexe2x80x94Crxe2x80x94Al alloys, oxide layers are formed with aluminum oxide initially forming an outer layer and chromium oxide an inner layer. In Fexe2x80x94Crxe2x80x94Si alloys, chromium oxide forms an outer layer with silicon oxide forming a thin inner layer (or scale) at the metal-oxide interface.
Aluminum and silicon also act to improve the oxidation resistance of stainless steels. It is thought that they act synergistically. U.S. Pat. No. 4,102,225 awarded to Michels on Jul. 25, 1978 provides a low-chromium containing alloy that is oxidation resistant. That alloy contains up to 4.5 percent aluminum, and up to 4.5 percent silicon with a combined silicon-plus-aluminum content of 2 to 7 percent.
Both aluminum and silicon concentrate in alloy substrates at the oxide scale/metal interface. However, silicon concentrates as metal while aluminum concentrates largely as an oxide. The aluminum oxide has been shown to disrupt the formation of the silicon oxide protective scale beneath the outer chromium layer discussed supra. The inventors believe that this disruption of the silicon layer compromises oxidation resistance, particularly at temperatures between 700xc2x0 C. and 800xc2x0 C.
Stainless steels exist which confer high oxidation resistance at or above 700xc2x0 C. However, these steels contain higher amounts of chromium and nickel, or additions of rare earth elements and other expensive additives. U.S. Pat. No. 4,063,935 awarded on Dec. 20, 1977 and U.S. Pat. No. 4,108,641 awarded on Aug. 22, 1978, both to Fujioka, et al., require a combination of silicon and rare earth elements to confer resistance above 1100xc2x0 C.
A need exists in the art for a low-cost oxidation resistant alloy and a method for producing an alloy that displays superior oxidation resistance. The alloy should contain relatively small amounts of chromium or nickel compared to the premium stainless steel alloys now available. Furthermore, the alloy should be resistant to oxidation at temperatures above 700xc2x0 C.
It is an object of the invention to provide an oxidation resistant alloy and a method for producing the alloy that overcomes many of the disadvantages of the prior art.
Another object of the invention is to provide an oxidation resistant alloy. A feature of the invention is the formation and presence of a continuous, defect-free layer of silicon-oxide and the absence of any aluminum. The absence of aluminum allows for the formation of higher amounts of silicon oxide films. Another feature is that the full austenitic microstructure of the alloy is maintained despite the silicon additions. An advantage of the invention is that the alloy has oxidation resistance characteristics at 800xc2x0 C. normally seen at 700xc2x0 C.
Yet another object of the present invention is to provide a method for producing a highly oxidation-resistant alloy. A feature of the method is subjecting an alloy to a pretreatment heating step at a first temperature to produce an oxide protective layer which is not produced at a lower temperature. An advantage of the invention is that the pretreatment step increases oxidation resistance of the alloy vis-a-vis untreated alloy and therefore extends the use of relatively low-cost conventional 18-8 type stainless steels in the temperature range between 700xc2x0 C. and the pretreatment temperature of 800xc2x0 C.
Briefly, the invention provides for a method for producing an oxidation-resistant alloy, the method comprising providing an alloy comprising iron, chromium, nickel, manganese, molybdenum, and silicon; and heating the alloy to 800xc2x0 C. prior to use.
Also provided is a fully austenitic alloy containing no aluminum, the alloy comprising iron, chromium, nickel, manganese, molybdenum and silicon.
The invention also provides a method for producing oxidation-resistant alloy for use at temperatures below 800xc2x0 C., the method comprising providing an alloy comprising iron, chromium, nickel, manganese, molybdenum, and silicon; and heating the alloy at a temperature and for a time sufficient to form a continuous oxide film on a surface of said alloy.